Engine Theory Question

[danhay]

When you open the throttle on an engine to go up a hill, but maintain constant speed and revs, does it use more fuel?

If so, how?

[danhay]

If you've got constant revs, then the same volume of mixture is being pumped the whole time. To use more fuel you've got to either make the mixture richer, or more dense?
A friend of mine reckons the extra power comes from increased pumping efficiency alone, but I can't help but think that it must use more fuel too?

[danhay]

It's not quite the same as a tap, cos it's not going from a closed system (pipe) to an open system (the air).

I reckon that although the mixture density in the inlet tract doesn't change, the final mixture density in the combustion chamber before the inlet valve closes is higher, because more fuel and air have been allowed in.

Sound reasonable???

[danhay]

kevinday said: In simple terms it takes more effort to lift a weight than to push it on the horizontal, so add the extra effort to move the car uphill to the effort required to maintain speed on the flat and you are working harder. For an engine to work harder it needs more fuel (and air) even at the same revolutions per minute

I think that makes sense

You can get more energy out of the engine without using any more fuel, simply by making it more efficient. Imagine you're pushing a car and the handbrake is slightly on. You can push it faster without any additional effort by releasing the brake. In this analogy the car is the engine and the brake is the throttle
...I hope that make sense!

Still think that you use more fuel - for the reasons stated earlier.

[danhay]

AJLintern said: Put simply, when you start to go up hill you will have to open the throttle more to maintain the same revs/speed, hence you are using more fuel.

Yes but can you explain the "hence" a bit more fully?